Mechanical face seals are used on various types of machines and equipment, such as pumps, compressors, and turbines which have a rotating shaft and a sealing chamber adjacent the shaft wherein the mechanical seal prevents leakage of fluid from the sealing chamber. Many such mechanical seals include a pair of adjacent seal rings which have opposing seal faces that define a sealing region therebetween to sealingly separate the sealing chamber from an exterior region. Typically, one of the seal rings is mounted on the shaft so as to rotate therewith while the other stationary seal ring is non-rotatably mounted on a seal housing.
Also, at least one of the rotating and stationary seal rings is axially movable. To maintain a seal between the opposed seal faces, the axially movable seal ring is axially loaded, such as by a spring or bellows, towards the other seal ring.
While the sealing region between the relatively rotatable seal faces defines the primary seal, secondary seals are provided between other adjacent components in the mechanical seal. For example, a secondary seal between the rotatable seal ring and the shaft or a shaft sleeve prevents migration of the sealed fluid therebetween, while a secondary seal between the stationary seal ring and a support element therefor prevents migration of the sealed fluid between these components.
As to such secondary seals, U.S. Pat. No. 5,813,674 defines a non-bellows seal arrangement wherein a secondary seal between a seal ring and a seal ring holder is a gasket which has a C-shaped cross section and a spring disposed within the gasket. Another seal arrangement having a spring energized plastic seal is disclosed in U.S. Pat. No. 6,116,610. However, these spring energized secondary seals can slide axially and thus, do not support the axial loads between the spring and the seal ring.
An inventive mechanical seal having an improved secondary seal arrangement overcomes disadvantages associated with known mechanical seals and is disclosed in U.S. Pat. No. 6,464,231 (Burroughs). The '231 patent is owned by the assignee of the present application, namely, Flowserve.
The gasket of the '231 patent relates to a mechanical seal having a spring loaded secondary seal which resiliently permits relative radial movement between a seal ring and a support element therefor, such as a bellows flange, and also supports axial loads between the seal ring and the support element.
In particular, a bellows type mechanical seal is provided wherein a non-rotatable, i.e. stationary, seal ring is axially movable and is axially loaded by a bellows which connects the stationary seal ring to the seal housing. The bellows includes a bellows flange at one end thereof which defines a support element that seats the stationary seal ring therein and has a secondary seal gasket therebetween. The opposite end of the bellows includes an annular adapter which seats within the seal housing and also has a secondary seal gasket therebetween. A further secondary seal gasket is provided between the rotatable seal ring and a support element therefor, namely a shaft sleeve.
The secondary seal gasket of the '231 patent is an annular gasket having a C-shaped cross sectional shape defined by upper and lower legs and an end wall. The upper and lower legs and the end wall define a gasket jacket in which an annular spring is received. The annular spring is disposed between the legs to press the legs radially away from each other into sealing engagement with opposed surfaces of the bellows flange and a gasket shoulder defined on the seal ring. The upper and lower gasket legs therefore can move radially relative to each other to accommodate relative radial motion between the seal ring and the support element, thus reducing seal face distortion due to vibrations, differential thermal expansion or contraction, and differential pressure expansion or contraction. Further, spring and pressure forces act on and through the gasket jacket to effectively seal and center the seal ring. The self-energized gaskets eliminate high radial forces and press fits required to seal a conventional rectangular flexible graphite gasket or high temperature metallic gasket. These extreme forces distort the seal ring lapped face.
Additionally, the lower gasket leg is confined axially between an end wall of the gasket shoulder and an opposing face of the bellows flange such that the axial loads applied on the bellows flange by the bellows are transmitted axially to the seal ring through the lower gasket leg. While the lower leg is resilient, the lower leg is constrained axially and therefore is stiff in that direction, particularly since a hydraulic pressure force between the gasket legs-stabilizes the lower leg and prevents buckling under axial loads.
The secondary seal arrangement and its application in a bellows type mechanical seal provides an improved seal having substantial axial load support while allowing radial motion of the parts which minimizes distortion of the faces and improves seal performance.
However, the gasket of the '231 patent typically is machined or formed from molded carbon filled PTFE composites or other polymer billets and typically is limited to about 500° F. during continuous duty, such as in a steam turbine seal. However, for larger steam turbines and other applications, the PTFE gasket is not suitable for higher operating temperatures such as above 800° F.
Typically, graphite is used to construct gaskets for higher temperature application. Known flexible graphite gaskets are conventionally formed from multiple wraps of a graphite ribbon material known commercially as Grafoil™. However, these gaskets have a square cross-section due to the wrapping process and conventional manufacturing techniques do not provide for a U-shaped cross-sectional shape like the gasket of the '231 patent. These square gaskets must be installed under compression or a predetermined interference to press fit. The forces required to seal this form of Grafoil gasket can cause distortion when used in conjunction with a rotating or stationary face.
The invention therefore relates to a graphite gasket which has a cross-sectional shape conforming generally to the U-shaped gaskets disclosed in the '231 patent so that the graphite gaskets disclosed herein perform the same functions while still providing a higher temperature capability which the graphite material provides. The graphite gasket of the invention is spring energized and the invention relates to the specific structure of the graphite gasket as well as the manufacturing method therefor.
Structurally, the graphite gasket is formed of multiple layers of graphite ribbon wherein a first section of the graphite ribbon is formed in a ring and then formed into an L-shape defined by an axial ribbon section which extends axially and a radial ribbon section which bends radially outwardly across the radial width of the finished gasket. The gasket also includes a second overwrap section wherein additional layers of graphite ribbon wrap are then wrapped about the axial ribbon section in a stack extending radially outwardly to the outer circumference of the radial ribbon section.
The finished gasket is provided with a U-shape defined by inner and outer axially extending legs and a radial end wall which spans the radial distance between the inner and outer legs and holds these legs together. The L-shaped graphite ribbon section extends axially along the inner gasket leg and then radially outwardly along the end wall to effectively define a barrier to fluid migration between the inner faces between the ribbon wrap.
In addition to this structural arrangement, the invention relates to the method of forming this gasket. In particular, the method comprises the steps of first wrapping Grafoil™ ribbon to provide a suitable diameter and then this initial ribbon section is formed into the L-shape. Thereafter, after forming this first ribbon section, additional overwraps of graphite ribbon are provided about the axial portion of the L-shaped section to the outermost diameter of the radial section. This provides an intermediate graphite ring having a substantially rectangular or square cross-section.
Then, this intermediate ring is molded by pressing a die axially into the end of the overwrap section to compress and shape the graphite ribbon into inner and outer gasket legs and define an interior annular groove in which a spring is seated.
The improved gasket thereby provides a graphite seal having a shaped cross-sectional configuration for use in high temperature applications. The graphite gasket may be installed into the mechanical seal disclosed in the '231 patent and thereby allows the mechanical seal of the '231 patent to be readily adapted to high temperature applications.
Other objects and purposes of the invention, and variations thereof, will be apparent upon reading the following specification and inspecting the accompanying drawings.
Certain terminology will be used in the following description for convenience and reference only, and will not be limiting. For example, the words “upwardly”, “downwardly”, “rightwardly” and “leftwardly” will refer to directions in the drawings to which reference is made. The words “inwardly” and “outwardly” will refer to directions toward and away from, respectively, the geometric center of the arrangement and designated parts thereof. Said terminology will include the words specifically mentioned, derivatives thereof, and words of similar import.